I. Field of the Invention
The present invention relates generally to dew point or humidity sensing and, more particularly, to dew point sensing using hydrophilic alkali metal salts in a heated equilibrium mode. The invention provides an improved, inexpensive, miniaturized, quick-response, dew point or humidity sensor characterized by long-term salt stability and low power consumption.
II. Related Art
The water absorption properties of a number of hygroscopic materials including alkali metal halide salts are known and have been employed as detection mechanisms for sensing dew point for many years. Thus, it is known that changes in water content or degree of water saturation in the salt changes the conductivity of the material which can be sensed between adjacent electrodes as a function of relative humidity. This approach has been implemented mainly using lithium chloride (LiCl) as the hygroscopic material. Commercial bulk-type dew point sensors have been fabricated with LiCl and, while functional, these sensors have had several undesirable characteristics. They generally have been relatively large and bulky leading to a corresponding undesirably slow response time. Those heated directly have needed an applied voltage level that is well above the electrochemical potential of the electrolyte/metal system, which led to poor stability and to shorter service life.
The principle that an hygroscopic salt exposed to ambient atmosphere and supplied with a regulated heat input can be caused to maintain a substantially constant predetermined resistance at equilibrium by absorbing and releasing moisture to the ambient atmosphere is known. It is further known that the temperature required to maintain this condition of equilibrium between the salt and the atmosphere that produces a given resistance is related to the dew point of the ambient atmosphere in a direct manner such that as the dew point or moisture content of the ambient atmosphere rises, the required equilibrium temperature likewise increases. In fact, it has further been found that the temperature to which the salt must be heated to maintain resistivity of the salt at its preselected level is, within narrow uncertainty limits, directly proportional to the water vapor content of the atmosphere.
Dew point measuring devices have been manufactured pursuant to this principle and in one such device a thermistor, or the like, embedded within the sensor unit of the hygrometer is utilized to produce a temperature signal that is in direct relation to the dew point of the atmosphere and, in this manner, the dew point is determined. Such a device is illustrated and described in U.S. Pat. No. 3,664,192. While this device has met with some success, there remains a definite need for a microminiaturized highly accurate dew point sensor which responds rapidly to changes in dew point, in which the hygroscopic salt is stabilized against washout and which can operate on low LiCl-electrode voltage and power input to achieve the desired service life.
Accordingly, it is an object of the invention to provide a low cost, microminiaturized dew point or humidity sensor characterized by a rapid response time, low power requirement and long life.
Another object of the invention is to provide a LiCl dew point or humidity sensor design that eliminates the problems of washout of the hygroscopic salt at high humidity, electrochemical degradation, and temperature measurement.
A still further object of the invention is to provide a LiCl dew point or humidity sensing device wherein retention of LiCl is accomplished by crosslinking polyvinyl alcohol.
A yet still further object of the invention is to provide a LiCl dew point or humidity sensing device which utilizes an etched diaphragm microsensor.